This invention relates to the field of agglutination assays to detect the binding of ligands, and particularly immunological binding (antigen and antibody binding) such as that involved in blood group serology ("immunohematology").
Blood group serology requires the determination of blood cell compatibility between a blood donor and a patient recipient before a transfusion or organ transplant involving the patient. Blood cell compatibility is determined by the non-occurrence of an immunological reaction between antibodies contained in the blood serum of a patient and antigens present on blood cells from the donor.
Many different blood group antigens are found on the surface of the red blood cells of every individual. These antigens, the products of inherited genes, exist in a unique combination in everyone except identical twins. Blood grouping is generally the process of testing red cells to determine which antigens are present and which are absent, normally utilizing antibodies to the antigen tested for.
Additionally, when a person does not have a particular red cell antigen on his red blood cells, his or her serum may contain an antibody to that antigen. Whether or not the antibody is present in the serum depends on whether the person's immune system has been previously challenged by, and responded to, that specific antigen or something very similar to it. For example, a person whose red blood cells are Type A, i.e., having "A" antigens on the red cells, will have anti-B antibodies in his or her serum. Thus, if such a person is given type B blood, an immunological reaction will occur with possible serious clinical consequences.
As an additional consideration, it should be noted that the human body is constantly exposed to antigens in pollens, food, bacteria and viruses. Some of these "natural" antigens are apparently so similar to human blood group antigens that they stimulate almost every susceptible person to produce antibodies. Thus, certain antibodies are expected in the serum of anyone whose red cells lack the reciprocal antigen. This is especially true of the ABO system. Accordingly, a second confirmatory test is often performed on the patient/donor sera. The test for expected antibodies of the ABO blood group system in sera is called "reverse" blood grouping.
Antibodies of the ABO blood grouping system are generally immunoglobulin M (IgM). These antibodies have ten antigen binding sites per molecule. The IgM antibody is large enough to span the distance between red blood cells, so that when they are centrifuged, the cells will be bound together in a lattice "cell-antibody-cell-antibody" and will remain clumped together in agglutinates. For example, if anti-A is added to blood group A or blood group AB cells and the mixture is centrifuged, the cells will remain in clumps when resuspended. With the same antibody, group 0 and group B cells will resuspend as individual cells. Agglutination caused by one antibody, such as an IgM antibody, is called direct agglutination.
The anti-Rh blood group reactions tend to give weaker agglutination which can be enhanced by the addition of high molecular weight polymers. Some anti-Rh antisera consist of immunoglobulin G (IgG) antibodies and will cause direct agglutination, if facilitated by the presence of high protein concentrations, such as 25% bovine albumin (often found in commercial anti-D reagent preparations).
Such facilitation is needed as IgG cannot easily span the distance between cells which tend to repel each other. Thus, it will bind to red cell antigens matching its specificity, but will not agglutinate such red cells as effectively as the larger IgM antibodies will. The presence of IgG antibody bound to a red cell is thus usually detected by the addition of anti-IgG which will cause the requisite agglutination, resulting in a lattice of "red cell-IgG/Anti-IgG/IgG-red cell".
Serum naturally contains IgG that will neutralize the anti-IgG antibody added to bind to red blood cells. Therefore, the serum must be removed before such anti-IgG is added to the cells. Tests for IgG bound to red cells in vivo are called direct antiglobulin tests. Tests for IgG bound to red cells in vitro are called indirect antiglobulin tests. Such antiglobulin tests are also called "Coombs" tests.
It is standard bloodbanking practice to test for A, B and D (RH.sub.o) antigens on a sample of a person's blood (and to perform tests for other antigens in selected cases), and to crosscheck the results by testing the person's sera to determine the acquired antibodies that might be present. The former is referred to as "forward typing" while the latter is referred to as "reverse typing". The results from each of these typing exercises have to agree.
Since the early 1900's, the general approach, known as the "Landsteiner" method, has been to add a patient's red blood cells to a standard laboratory test tube containing a blood group antibody (suchas Anti-A or Anti- B), mix to allow antibody/antigen binding reactions to take place, and then to centrifuge. If the antigen tested for is present, the antibody/antigen binding will have taken place resulting in agglutination of the patient's red blood cells. The test tube is manually shaken to dislodge the centrifuged button of "clumped" cells at the bottom. A subjective determination is then made as to whether or not the dislodged cells are "clumped", and to what extent.
During the mid-1900's, attempts were made to simplify this technique to minimize the subjective nature of the test and to reduce mistakes. It was recognized that a somewhat permanent record of the results of compatibility testing could be had by employing wettable, either non-absorbent, or in some cases absorbent, test slides or test cards having the requisite immunochemical reagents on at least a portion of their surfaces. U.S. Pat. Nos. 2,770,572, 2,850,430, 3,074,853, 3,272,319, 3,424,558, 3,502,437 and 3,666,421, and European Patent Application #0 104 881-A2 depict select examples of such test cards and related apparti.
More recently, techniques to increase the sensitivity and accuracy of certain antiglobulin testing, such as the so-called "Coombs" test, have been developed. U.S. Pat. No. 4,435,293 to Graham et al. describes a bloodtyping system (Simwash.sup.R) which uses a system of test tubes that eliminates the washing steps of the original Coomb's method, as the Simwash.sup.R system provides "self-washing" of the red blood cells. Antiglobulin tests, such as that just described, require that the red blood cells be free of their serum, which contains unbound IgG antibodies. With the Simwash.sup.R system, the dense cells are centrifuged into a column, while the less dense serum remains at the top.
European Patent Application #0 194 212 describes a blood compatibility testing system using gel in a column, such gel being in particular, Sephadex.sup.R which is a 3-dimensional network or matrix of dextran chains cross-linked with epichlorhydrin (product of Pharmacia Fine Chemicals, Uppsala, Sweden and Piscataway, N.J.), that catches agglutinated red blood cells and allows unagglutinated cells to pass through to the bottom.
One of the drawbacks in the use of the last-mentioned system, and especially in the manufacture of such a test system using gel as a medium, is that gels such as Sephadex.sup.R have to be settled prior to insertion into a test tube to get rid of the so-called "fines" which are contaminating small molecular weight compounds or fragments that may interfere with the separation capacity the gel. Gel fines have historically been known to clog chromatographic separation columns.
A gel also has to be swelled prior to use and in order to accurately calculate the amount of reagents to be used in conjunction with the test system. Calculations of requisite amounts of reagents, buffers and the like have to take into account the gel swelling dilution factor. Additionally, some of the reagent is lost as it is absorbed by the porous gel and not available for binding to the analyte tested for. Greater quantities of reagent must be added to compensate for this loss. The gel is also inherently more fragile to mechanical handling, and can break apart during the swelling process, causing an uncontrollable variation in gel particle size.
Another drawback one suffers in the use of Sephadex.sup.R is that it is very fragile to temperature extremes, and tends to dry out or break apart, causing stability, shipping and storage problems, which include shrinkage, and concurrent alterations in test properties. The breakage can result in such above-mentioned fines that alter or restrict the normal flow of the blood cells through the material. An additional shipping and storage problem is occasioned by the inability to freeze Sephadex.sup.R -containing test devices.